MONDAY, Nov. 2, 2015 (HealthDay News) -- Nearly all U.S. kids under age 4 have used a mobile device such as a tablet or smartphone, and they are using them at earlier and earlier ages, a new study finds.

The study of 350 children in a low-income, minority community suggests that an income-based "digital divide" is shrinking. Parents surveyed said tablets are the most popular mobile devices for children, and kids as young as 1 use them more than 20 minutes a day on average.

"Access to, familiarity with and skill using mobile devices are the first steps in achieving digital literacy," said one of the study's authors, Dr. Matilde Irigoyen, chair of pediatrics and adolescent medicine at Einstein Medical Center in Philadelphia.

However, she added that actual social engagement with parents, along with parental guidance, are "critical" to help kids "integrate digital technology into family life."

The study was published in the Nov. 2 online edition of the journal Pediatrics.

One expert familiar with the study findings called the trend "disturbing," because it suggests that some parents might be using technology as a surrogate babysitter.

"Parents in this study admitted to using mobile media for their children to keep them quiet or entertained in public places or in place of the interaction at bedtime," said Dr. Danelle Fisher, vice chair of pediatrics at Providence Saint John's Health Center in Santa Monica, Calif. "Children need parental interaction for many reasons and this trend is, overall, worrisome."

For the study, researchers gave a questionnaire to parents of 350 children between 6 months and 4 years of age who visited a Philadelphia pediatric clinic in October and November of 2014. The parents answered questions about the children's use of television, mobile devices, computers and video games.

Almost every household (97 percent) had a TV, 83 percent had tablets, and 77 percent had smartphones. Just over half had video consoles (56 percent), a computer (58 percent), and Internet access at home (59 percent).

The older the children were, the more likely they were to have their own technology. By age 4, about three-quarters of youngsters had their own mobile device, and half had their own TV. All but 3 percent had used a smartphone or tablet, the study found.

"We were not surprised to see infants and toddlers using the mobile devices; we saw that in the clinic every day," Irigoyen said. "But we were very surprised to see how often the children used the mobile devices, how many of them owned a personal device, how many could use the device without assistance, and how many engaged in media multitasking."

More than four out of 10 children under age 1 and 77 percent of 2-year-olds used a mobile device every day to play games, use apps or watch videos, the investigators found.

Almost half of 4-year-olds and one-quarter of 2-year-olds needed no help using a smartphone or tablet, the findings showed.

The parents' education and the child's gender and ethnicity did not play any role in whether or not the child owned a mobile device.

While 28 percent of parents said they used a mobile device to get their child to go to sleep, 70 percent did chores while their child played. A majority reported letting their child play with a mobile device to keep them calm in public places or during errands.

But, Fisher said, "mobile media cannot take the place of parenting." She added that the use of tablets and smartphones "in place of human interaction and instead of other sources of information or play time, such as reading books, is disturbing."

Yet about half the parents had downloaded apps on their smartphones, and they said at least half were for their children, especially educational, entertainment and video apps.

The study found that children still spent more time watching TV than using any other single screen, an average 45 minutes a day. They also averaged 27 minutes a day watching videos or TV shows on mobile devices and 22 minutes using apps.

"Our findings do not address the impact that mobile media devices have on children and their families," Irigoyen said. "They simply describe the pervasive nature of digital exposure and the patterns of use."

Fisher pointed out that "some interactions can be beneficial and some detrimental." But, she added, "there are lots of studies showing that mobile media devices will interfere with healthy sleep practices. Topics such as obesity, academics and attention are more complex."